This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Recent developments of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) facilitate accessing local Internet Protocol (IP)-based services in various places, such as at home, office, or public hot spots, or even in outdoor environments. One of the important use cases for the local IP access and local connectivity involves a so-called D2D communication mode, wherein UEs in close proximity (typically less than a few tens of meters, but sometimes up to a few hundred meters) communicate with each other directly.
As discussed in Reference 1 (G. Fodor, G. Mildh, E. Dahlman and S. Parkvall, “Network Assisted D2D Communications in LTE”, FRA Seminar material, December 2010), because D2D UEs are much closer to each other than cellular UEs that have to communicate via at least one cellular access point (e.g., an eNodeB (eNB)), the D2D communication enables a number of potential gains over the traditional cellular technique, including capacity gain, peak rate gain, and latency gain.
The capacity gain may be achieved, for example, by reusing radio resources (e.g., Orthogonal Frequency Division Multiplexing (OFDM) resource blocks) between D2D and cellular communications and by reducing the number of links between UEs from two to one and accordingly reducing the radio resources required for one link. The peak rate gain directly results from the relatively short distance between D2D UEs and the potentially favorable propagation condition therebetween. The latency gain is also a direct result of the single relatively short link between D2D UEs.
FIG. 1 illustrates an example of a mixed cellular and D2D network according to the prior art, wherein UE 101 is a cellular UE which communicates via an eNB 110, whereas UEs 102 and 103 are D2D UEs which communicate with each other directly. UE 101 may be also a D2D UE, which communicates with other D2D UEs such as UE 103. As Reference 1 suggests, in such a mixed cellular and D2D network, D2D communications share radio resources with UL cellular communications, and a Time Division Multiplexing (TDM) is used for dividing resources between cellular communications and D2D communications, so as to avoid collision.
FIG. 2 illustrates an example TDM based cellular/D2D resource division. As shown in FIG. 2, cellular regions and D2D regions are separated in time domain within a cell. It should be noted that the resource division may be known by each D2D UE via system broadcast information. Within the allocated D2D resource pool, each UE may select resource for D2D transmission. However, since it is possible that different cells are asynchronous with each other, the D2D resource pool within cell 1 could collide with the cellular resource pool in cell 2, thereby leading to inter-system interference between the two cells.
However, if there is no inter-cell synchronization (if considering intra-cell synchronization can be easily achieved, between intra-cell sectors), there would be still inter-system (cellular and D2D system) interference among neighboring cells.
FIG. 3 illustrates simulations for cellular capacity versus D2D capacity via the TDM solution in different scenarios, where Case 1: 500 m ISD, uniformly outdoor; Case 2: 1732 m ISO, uniformly outdoor.
From the simulations, it can be seen that the inter-system interference would cause dramatically drop of cellular throughput in dense network scenario (500 m ISD, uniformly outdoor UE deployment).